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Bias on Ampeg BA115 too low cause occasional random-ish noise?

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  • #1

    Bias on Ampeg BA115 too low cause occasional random-ish noise?

    I looked at my friends BA115 bass amp in the past and fiddled with checking input jack contact tightness and cleaned the jack a bit.
    The noise had sounded like when you have bad connection at the input, whether jack or cord. Tht annoying static pop thing.
    It also didn't seem nearly loud enough for a 220 watt amp.

    Anyway, when I got done, I couldn't make it cut up again so I sent it home with him.
    He uses it about once a week for about 3-4 weeks without trouble and then it unexpectedly starts making that noise again, doesn't get nearly as loud as it should and lost a lot of treble.

    I open it up and check VCC and lower voltage to ICs. (whatever we call that)
    I get +/- 59v and +/- 17V.
    Schematic shows 50 and 16.

    I move on and decide to check the bias because the test and adjustment is straightforward.
    Bias should be between 15-20 mV.
    I find it to be 2.7 mV, so I crank it up to 17-18mV.

    I plug it back up to the cab and pump tunes into it.
    Loud and sounds great.

    Could it have been that very low bias causing problems or is it a matter of time before the problem comes back
    I've been playing music for about an hour now and will go a little longer before I have to move onto something else outside the shop.
    Tags: None
  • #2
    No, the bias is not involved. I doubt that lower bias would be audible in any way. But about the only symptom of too low bias on an amp like this would be increased crossover distortion.

    Doesn't that amp have a headphones jack? Check the resistance n the speaker cutout contacts on that.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      No, the bias is not involved. I doubt that lower bias would be audible in any way. But about the only symptom of too low bias on an amp like this would be increased crossover distortion.

      Doesn't that amp have a headphones jack? Check the resistance n the speaker cutout contacts on that.
      Would the very low bias explain why i always thought this amp was way too quiet for its rated output power?

      This one does have a headphone out. I'll check the jack.

      Comment

      • #4
        Again, no. A very cold amp has more crossover distortion. The output level would be diminished only to the extent that little crossover notch removes anything, wwhich is to say virtually nothing.

        If your aamp has weak output, then I usually suspect either an impediment in the signal opath, like a bad phones jack or a bad loop return jack.

        Try to determine if the problem is in the power amp or the preamp. Put a line level signal into the CD input jcks and see if you can drive the output to loud. Also there is a limiter circuit at the power amp input stages, it could be compromised.

        Also, do you have a BA115HP instead of a BA115?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Oops. Yes it is the HP model. Can't believe I left that out of the thread title.
          Probably because I own the non-HP model.

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          • #6
            Originally posted by Enzo View Post

            If your aamp has weak output, then I usually suspect either an impediment in the signal opath, like a bad phones jack or a bad loop return jack.

            Try to determine if the problem is in the power amp or the preamp. Put a line level signal into the CD input jcks and see if you can drive the output to loud. Also there is a limiter circuit at the power amp input stages, it could be compromised.
            Resurrecting an old thread, but it's still relevant. I'm finally back around to this Ampeg B115HP after pushing it aside and successfully fixing other amps and issues.
            Tonight I plugged my phone into the CD input and played some music. it got quite loud with the phone volume up and the master cranked up.

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            • #7
              Originally posted by Enzo View Post

              Doesn't that amp have a headphones jack? Check the resistance n the speaker cutout contacts on that.
              It does have headphone jack. when i check across the terminals I got vary resistance, but up to 4 ohms across the front 2 contacts and about .4 on the rearmost contact.

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              • #8
                When you say the rear most contact I am thinking that is the ground contact and the other two are the tip & ring. The audio signal passes through the tip & ring contacts and the jack switches can get worn out/dirty. One that I worked on I was able bend the contacts so they would be closed again. So you are measuring some resistance on what should be a closed jack switch contact.
                When the going gets weird... The weird turn pro!

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                • #9
                  The one I'm referring to as "rearmost" would be the one farthest away from the faceplate, so that would be the tip in this case.

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                  • #10
                    If I recall the circuit, the "ground" contact on the phones jack shorts across a resistor. Plug phones in and the resistor is now in series with the output, reducing it for the phones. If that contact does not close electrically, then the resistance is in series with your speaker, and thus reduced or missing sound. Anything over a half ohm on jack contacts constitutes a problem. It means the contacts need to be cleaned and/or reformed.

                    If the CD input comes out loud, then the power amp is working. I forget, is there a FX loop or insert jack? If so, I'd clean that. I'd also see if the send signal there is strong or diminished.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      no FX loop on this one. I cleaned the jack and it get loud (vague i know), but I have to push the gain and master up to about 8 to get a volume i think i should be having at about 1/2 way.

                      Without having actually measured the output, or signal at various test points on the schematic (i know, shame on me) it's hard to know if its operating at full capacity.

                      30 years of playing bass tell me 2 things, 1) an amp rated at 220 watts *should* be louder 2) Ampeg bass amps (100-220 watt versions) never seem as loud as similarly rated "other brands"

                      I'm looking at the test parameters on the schematic. 1 kHz sine wave with gain at volume at 10. Please tell me i can do this with speaker unhooked?

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                      • #12
                        Yes it's solid state.
                        KB

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                        • #13
                          I suppose if it did need a load it would stated so, as a resistive load. I need to get my load bank wired up anyway, but not for this exercise

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                          • #14
                            Actually, I can't think offhand of any schematic that warned we MUST use a load. Solid state amps don't ccare, but tube amps MUST have a load. That is something any tech knows, but I don't see it on schematics.
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment

                            • #15
                              Originally posted by Kenrod View Post
                              no FX loop on this one. I cleaned the jack and it get loud (vague i know), but I have to push the gain and master up to about 8 to get a volume i think i should be having at about 1/2 way.

                              Without having actually measured the output, or signal at various test points on the schematic (i know, shame on me) it's hard to know if its operating at full capacity.

                              30 years of playing bass tell me 2 things, 1) an amp rated at 220 watts *should* be louder 2) Ampeg bass amps (100-220 watt versions) never seem as loud as similarly rated "other brands"

                              I'm looking at the test parameters on the schematic. 1 kHz sine wave with gain at volume at 10. Please tell me i can do this with speaker unhooked?
                              There are a number of versions of this amp, circuit wise.

                              The test of the power amp shows a 26.3 Vrms ac output signal.
                              So, if you run this amp in to an 8 ohm load, that translates to an 86.5 watt amp.

                              BA115HA_v3.zip

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